<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <div>显示的内容. </div>
    <style>
        div{
            box-sizing: content-box; 
            box-sizing:border-box;
            border: green 10px solid;
            width: 100px;
            height: 50px;
        /* 外边距不影响 */
            margin: 30px;
    }
    </style>
</body>
</html>